You kind of answered the question yourself in the OP.

@ bold, I don't think a parallel can be drawn between Animal Crossing & Wii Sports/Fit.  Animal Crossing had been a thing since 2001, a long time before the WIi hit market.  It's more of a core Nintendo IP at this point.

A better comparison would be comparing Wii Sports/Fit to some of the other fads that took off on DS, such as Nintendogs or Brain Training.  Just like Wii Sports/Fit these led to a lot of imitators which saturated the market, and by the time we got new instalments on the 3DS no-one was interested anymore.

I mean, Nintendogs 3DS has sold pretty well for what it is, but it's nowhere near the numbers the series put out on DS.  I also suspect it's no longer a system seller, but rather a game people picked up with their 3DS because there wasn't much else in the first 6 or so months.

The new Brain Training, I believe, completley bombed by all accounts.

@ underlined, that's exactly the problem.  The people who buy Just Dance/Zumba Fitness already have machines that play Just Dance/Zumba Fitness.  Are they going to go out and buy another machine for £200+ just so they can play the latest iteration?

I'd hazard a guess that the people who are buying these type of games aren't the type to loyally follow a franchise.  They probably weren't even console gamers before the Wii came along.  And it's going to take a hell of a lot of convincing them that they need a Wii-U so they can continue shaking motion controls at a screen.

Are these easily replaceable sport, fitness or dance games with no story like Wii/Kinect Sports, Wii/Zumba Fit or Just Dance?

Mario Kart is, yes. I also don't think anyone buys AC for its "story" really, it's just more of the same too.

Honestly?

Because of Wii Fit and Wii Sports. The non-gamers who wanted those titles already bought them. A good number of those customers will be content with the Wii and will have no desire to upgrade, while another segment will have moved on from console gaming completely. Such is the difference between traditional gamers and those who bought the Wii because they played it at a party or they wanted to lose 15 pounds. They're not going to spend $300+ dollars for another console they'll rarely use.

Even besides that, you have the confusing identity of the Wii U. They'd have to run some massive advertising to even communicate to many people that the gamepad isn't the only means of control.

i know a few woman who bough a wii just because of wii fit so at least my own experience say yes, it was a system seller. 

but no, i don't think that they will also buy a wii u with a wii fit u. they lost the interest in this concept or are still fine with their wii and wii fit. 

there's no wii sports in sight. I don't know about wii fit U, but wii party U may sell really well. being $49 while coming with a wii remote + and making great use of the gamepad, it can be a hit.

The only thing we can really go off is attach ratio, when comparing Wii Sports and Wii Fit.

Wii Sports Resort has about a 30% attach ratio. Wii Fit and Wii Fit Plus both have around a 20% attach ratio. If the Wii U goes on to sell 30 Million units lifetime, then I can agree that a Wii U Sport would sell around 9-11 Million (Though this may be skewed because of WS bundling). On the other hand, Wii U Fit would have to be around 6-8 Million.

They will sell some systems, but to expect anything at all like Wii-level sales is wrong. There is more competition now. The Wii U isnt the Wii. Nintendo arent in the position they were in 6 Years ago.

AC is still selling amazing well which proves that the casual are still interested in this ip not only the Nintendo fans. BT, Nintendogs have a lot of ripp-offs on smartphones while the only games that can compete against WS are on other expensive consoles and those games didn't have the same kind of success like MK clones never overshadowed the original. Speaking of WF, it really has no competition, Zumba being the closest one.

So the situation seems very different to BT and ND and at least there is no proofs compelling enough to claim that these particular ips will have the same fate as BT and ND and not continue to sell really well like other casual ips like AC, TC or even MK.

I thought a strong point for the Animal Crossing series are the cute characters with their spleens and funny back stories, witty dialogs and amusing situations... similar to the success of The Sims.

Mario Kart appeales with well known characters of all strong Nintendo franchises (Mario, Luigi, Donkey Kong, Peach...)

Wii Sports has neither of it, only boring Miis and no story.

Predicting what games casual gamers will buy is implying you know what casual gamers want. And we never experienced such gamers like the ones interested in home consoles like the Wii besides maybe the first NES and PlayStation. I argue that none of us still really know how they think.

Are you claiming you know how casual gamers think?